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Fire down below

7 August 2004

By Stephen Battersby

SOON we will be able to see into the centre of the Earth. Giant detectors will let us look down through the crust, the rocky mantle and the iron core. And there, thinks Marvin Herndon, we will see an enormous nuclear reactor, an 8-kilometre ball of fissioning uranium and plutonium.

Herndon is an independent geophysicist based in San Diego, California. For years, his georeactor idea has been shrugged off as a wild guess, but now physicists are devising machines that will find out whether Herndon is right or wrong. Either way, they should answer a question that no one has yet managed to resolve&colon; what is generating the intense heat that exists in the inner core of the Earth?

Though controversial, Herndon’s answer is simple enough. For starters, uranium is an exceptionally heavy element, so when the Earth was young and molten, he reasons, most of its uranium would have sunk to the centre of the planet (see “A heart of uranium?”). There would have been enough to gather into a ball several kilometres across, forming a huge natural nuclear fission reactor.

We know natural reactors can exist. One burnt on Earth about two billion years ago at Oklo in Gabon, west Africa, when uranium in the crust became concentrated enough to trigger a controlled chain reaction. Could the same thing happen on a vast scale at the core?

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That could certainly explain another mystery&colon; the fact that our planet is producing more heat than anyone can account for. Most physicists believe the Earth’s magnetic field is …